CSE 486/586 CSE 486/586 Distributed Systems Concurrency Control Steve Ko Computer Sciences and Engineering University at Buffalo

CSE 486/586 Recap Strict execution of transactions? –Delay both their read and write operations on an object until all transactions that previously wrote that object have either committed or aborted Two phase locking? –Growing phase –Shrinking phase Strict two phase locking? –Release locks only at either commit() or abort() 2

CSE 486/586 CSE 486/586 Administrivia PA3 deadline: 4/11 (Friday) Midterm: Next Monday 3

CSE 486/586 Distributed Transactions Transactions that invoke operations at multiple servers 4 T A Y Z B C D T T1T1 T2T2 T11 T12 T21 T22 A B C D F H K Flat Distributed Transaction Nested Distributed Transaction X

CSE 486/586 Coordinator and Participants Coordinator –In charge of begin, commit, and abort Participants –Server processes that handle local operations 5 T A Y Z B C D X join Coordinator Participant Coordinator & Participants

CSE 486/586 Example of Distributed Transactions 6.. BranchZ BranchX participant C D Client BranchY B A participant join T a.withdraw(4); c.deposit(4); b.withdraw(3); d.deposit(3); openTransaction b.withdraw(T, 3); closeTransaction T =openTransaction a.withdraw(4); c.deposit(4); b.withdraw(3); d.deposit(3); closeTransaction Note: the coordinator is in one of the servers, e.g. BranchX

CSE 486/586 Atomic Commit Problem Atomicity principle requires that either all the distributed operations of a transaction complete, or all abort. At some stage, client executes closeTransaction(). Now, atomicity requires that either all participants (remember these are on the server side) and the coordinator commit or all abort. What problem statement is this? Consensus Failure model Arbitrary message delay & loss Crash-recovery with persistent storage 7

CSE 486/586 Atomic Commit We need to ensure safety in real-life implementation. Never have some agreeing to commit, and others agreeing to abort. First cut: one-phase commit protocol. The coordinator communicates either commit or abort, to all participants until all acknowledge. What can go wrong? Doesn’t work when a participant crashes before receiving this message. Does not allow participant to abort the transaction, e.g., under deadlock. 8

CSE 486/586 Two-Phase Commit First phase Coordinator collects a vote (commit or abort) from each participant (which stores partial results in permanent storage before voting). Second phase If all participants want to commit and no one has crashed, coordinator multicasts commit message If any participant has crashed or aborted, coordinator multicasts abort message to all participants 9

CSE 486/586 Two-Phase Commit Communication 10 canCommit? Yes doCommit haveCommitted Coordinator 1 3 (waiting for votes) committed done prepared to commit step Participant 2 4 (uncertain) prepared to commit committed statusstepstatus

CSE 486/586 Two-Phase Commit To deal with server crashes Each participant saves tentative updates into permanent storage, right before replying yes/no in first phase. Retrievable after crash recovery. To deal with canCommit? loss The participant may decide to abort unilaterally after a timeout (coordinator will eventually abort) To deal with Yes/No loss, the coordinator aborts the transaction after a timeout (pessimistic!). It must announce doAbort to those who sent in their votes. To deal with doCommit loss The participant may wait for a timeout, send a getDecision request (retries until reply received) – cannot abort after having voted Yes but before receiving doCommit/doAbort! 11

CSE 486/586 Problems with 2PC It’s a blocking protocol. Other ways are possible, e.g., 3PC. Scalability & availability issues 12

CSE 486/586 Summary Increasing concurrency –Non-exclusive locks –Two-version locks –Hierarchical locks Distributed transactions –One-phase commit cannot handle failures & abort well –Two-phase commit mitigates the problems of one-phase commit –Two-phase commit has its own limitation: blocking 13

CSE 486/ Acknowledgements These slides contain material developed and copyrighted by Indranil Gupta (UIUC).